Prostate cancer is mainly developed in men 60 years or older. The prostatic cancer has become the second leading cause of cancer-related death, after lung cancer, for men in the American and European countries. The incidence of prostate cancer has increased since 1975, and one of the reasons is the spread of diagnosis using the measurement of the prostate specific antigen (hereinafter referred to as PSA). Early cancer which is difficult to detect by a conventional digital rectal examination has been found by the measurement of PSA.
PSA is a protein secreted in a glandular cavity of the prostate from glandular cells of the prostate. PSA is expressed in the prostate tissue-specifically, but not cancer-specifically. Thus, it is known that the PSA is increased in the benign diseases such as prostatic hypertrophy and prostatitis other than the prostate cancer.
At present, the PSA assay widely used is total PSA assay wherein both complexed PSA, in which PSA bound to α1-antichymotrypsin (hereinafter sometimes referred to as PSA-ACT), and free PSA can be measured. When the measured value of total PSA of a subject is not less than 10 ng/mL, the possibility of prostate cancer is 50% or more of the subject. Twenty five percent of patients having a total PSA value of 4-10 ng/mL are prostate cancer patients, and 15% of patients having a total PSA value of 2-4 ng/mL are prostate cancer patients. The range of 4-10 ng/mL of total PSA is refered to as gray zone. Even in the case of patients having prostatic hypertrophy, there are many patients having a total PSA value of the gray zone. For this reason, the development of a method for analyzing PSA, which can distinguish between prostate cancer patients and prostatic hypertrophy patients, is desired.
Measuring the ratio of free PSA to total PSA is carried out in order to distinguish prostate cancer from prostatic hypertrophy in patients having total PSA values in the gray zone. It has been reported that the ratios of free PSA to total PSA in sera of prostate cancer patients is lower than that in normal sera. Free PSA value is measured by the ELISA method for free PSA. Then the ratio of free PSA value to total PSA value (hereinafter sometimes referred to as “free/total PSA ratio”) is calculated. When the value of a free/total PSA ratio is not more than 25%, there exists tumors in the prostate at the high frequency. However, in samples of the gray zone, the probability of prostate cancer is 56% in cases having a free/total PSA ratio of 0 to 10%, the probabilty of prostate cancer is 28% in cases having a free/total PSA ratio of 10-15%, the probabilty of a prostate cancer is 20% in cases having a free/total PSA ratio of 15-20%, and the probabilty of a prostate cancer is 16% in cases having a free/total PSA ratio of 20-25%. Thus, even if a free/total PSA ratio is used, it is not easy to distinguish prostate cancer from prostatic hypertrophy.
In the case of the patients having more than 10 ng/mL of total PSA and the patients having 4-10 ng/mL (gray zone) of total PSA and not more than 25% in a free PSA/total PSA ratio, a biopsy of prostate grand is performd for a definitive diagnosis of the prostate cancer. However, in the latter patients, prostate cancer can be detected at the possibility around 30%. Therefore, patient is placed an excessive burden. For this reason, the development of a method for simply and easily distinguishing prostate cancer from prostatic hypertrophy has been desired.
It is known that PSA is a glycoprotein having one asparagine-linked carbohydrate chain (hereinafter referred to as an N-glycan chain), and the PSA from prostate cancer patients has higher-branched complex type of N-glycans. Further, it has been considered that PSA of prostate cancer patients might have a cancer-specific carbohydrate chain. For example, an N-glycan chain of PSA secreted from LNCaP cells derived from a prostate cancer was analyzed by using a mass spectrometer. As a result, it was reported that the N-glycan chain has a high content of N-acetylhexosamine (HexNAc) and fucose, and less sialic acids as compared to an N-glycan chain in PSA of normal seminal fluid (non-patent document 1). However, the sugar chain structure of PSA from the LNCaP cells is different from that in serum PSA in prostate cancer patients, because the carbohydrate chain in PSA of the LNCaP cells contains less sialic acid residues. Therefore, the charactes of PSA from LNCaP cells were not considered to be the same as those of PSA from prostate cancer patients.
Also, Oyama et al. found that N-glycans of PSA in the prostate cancer patient serum contained sialic acid α(2,3) galactose residues (Sialic acid α 2,3 Gal-R), and that more sialic acid α(2,3) galactose residues were linked to PSA of prostate cancer patient serum as compared to PSA of prostatic hypertrophy patient serum (patent Reference 1 and non-patent Reference 2). Further, a method for distinguishing prostate cancer from prostatic hypertrophy has been found by binding PSA from the prostate cancer patient serum to Maackia amurensis agglutinin (hereinafter referred to as MAA) which can specifically bind to the sialic acid α(2,3) galactose residues, and measuring the ratio of MAA-bound PSA to total PSA (patent reference 1 and non-patent reference 2). However, besides PSA, α1-antichymotrypsin has the sialic acid α(2,3) galactose residues, and therefore α1-antichymotrypsin can also bind to MAA. Thus, in the case of the PSA bound to α1-antichymotrypsin, i.e. PSA-ACT, PSA having sialic acid α(2,3) galactose residues cannot be separated from PSA lacking sialic acid α(2,3) galactose residues. Thus, it is necessary to measure free PSA in order to distinguish a prostate cancer patient from a prostatic hypertrophy patient.
Tajiri et al. have compared the sugar chain structures of PSA from two prostate cancer patient sera to that of PSA in normal seminal fluid by using mass spectrometry. It has been reported that PSA of the prostate cancer patient serum is sialylated and fucosylated (non-patent reference 3). However, it has not been reported that a prostate cancer patient can be distinguished from a prostatic hypertrophy patient by the analysis of carbohydrate chains other than the sialic acid α(2,3) galactose residues.